A sandwich and a piece of fruit will probably be in the 400-800 calorie range. It’s absolutely enough food for 1/3 meals. Especially if the kid is also eating snacks
the official US guidelines for school provided lunch range from 550–850. that's a bit lower than i would choose based on daily needs especially for older kids. breakfast and lunch should be more than 1/3rd each. dinner should be less than 1/3rd.
i am from austria. a sandwich here is considered a snack. not a meal.